Area Forecast Discussion
National Weather Service San Juan PR
339 AM AST Tue Jun 30 2020
Passing early showers will affect the islands due to a induced
surface trough crossing the area. A tropical wave will approach
the area by Wednesday and Thursday, increasing moisture and shower
activity over the islands. In general, an unsettled weather
pattern is expected to prevail through the next few days.
.SHORT TERM...Today through Thursday...
Broad upper level trough will linger across the region through
Wednesday, then gradually weaken its hold by Thursday at it lifts
north and east of the area. This will maintain unstable conditions
aloft during the period. An induced low level trough will
continue to cross the forecast area today, with the low level
winds forecast to become more southeasterly during the afternoon
as the trough shifts farther west of the region.
The combination of the two aforementioned features will aid in
enhancing passing shower activity over the regional waters during
the rest of the early morning hours. Periods of moderate to locally
heavy rains of short duration will affect parts of the north and
east coastal areas of the islands from time to time. During the rest
of today, available moisture and the favorable conditions aloft,
along with daytime heating will again favor enhanced afternoon
convection over parts of the islands. However, most of the activity
should be focused over the central interior and northwest sections
of Puerto Rico and on the west-end of the U.S.Virgin Islands,as the
steering winds become more southeasterly. Periods of moderate to
locally heavy rains and isolated thunderstorms will be possible over
these areas, as well as around parts of the San Juan metro where
showers may develop later in the afternoon.
On Wednesday, lesser moisture convergence and instability aloft is
forecast as both the upper trough and low level trough will weakens.
However, brief early morning passing showers will still be possible
followed by locally and diurnally induced afternoon convection over
parts of the islands. By late Wednesday into Thursday , recent model
guidance was less bullish with moisture transport across the region,
but still suggest increasing moisture and instability accompanying a
tropical wave which is forecast to quickly cross the region on
Thursday. This along with the proximity of the upper trough just
north of the region should favor fairly active weather condtions
with better potential for shower and thunderstorm activity in and
around the islands. Urban and small stream flooding as well as
ponding of water on roadways and in poor drainage areas will
therefore remain possible in isolated area especially during the
afternoon hours. In general, an overall moist and unstable weather
pattern is expected to prevail across the forecast area for the
short term period.
.LONG TERM...Friday through Wednesday...
As the tropical wave depart from the islands, a drier air mass,
along with some Saharan dust is expected to filter in from the east
on Friday. At the upper levels, a low pressure system positioned
over Hispaniola will add some instability, with 500 mb temperatures
at around -7 degrees. Therefore, there will be a potential for
showers and thunderstorms, especially during the afternoon hours
over northwestern Puerto Rico. This activity is not expected to be
For the weekend, moisture increases, as an easterly perturbation
advances over the islands. With the upper level low still in place
west of Puerto Rico, a similar pattern should be expected, but with
better moisture as the surface, more frequent passing showers should
be moving over the islands, followed by the usual afternoon activity
affecting the western half of Puerto Rico.
For the workweek, a mid level ridge will develop north of the
area, while the upper level low moves away. Even though this will
result in a reduction of instability aloft, with 500 mb temperatures
warming up to -5 to -3 degree Celsius, a series of tropical waves
will be advancing from the east, supplying low level moisture.
Therefore, the potential will still exist for the development of
shower activity over the area each day.
Prevailing VFR durg prd but VCSH psbl at TIST/TISX/TJSJ/TJBQ til
29/14Z. SCT OCNL BKN cld lyrs nr FL025...FL050 w/WDLY SCT passing
SHRA ovr regional waters and en route btw islands. SHRA/Isold TSRA
psbl at TJMZ/TJBQ fm 30/17Z-30/22Z with VCSH psbl at remaining
terminals. SFC wnds calm to lgt/vrb bcmg fm E to SE at 10-20 kts aft
29/14z with sea breeze variations. L/lvl wnds fm E btw 10-15 kts BLO
Generally tranquil marine conditions are expected today with seas
at 5 feet. However, by mid-week, seas will become choppy as a
tropical wave crosses the islands. This will increase the
potential of thunderstorms as well. For the beachgoers, a moderate
rip current risk exist for some of the local beaches.
.PRELIMINARY POINT TEMPS/POPS...
SJU 92 79 90 79 / 30 40 40 40
STT 91 80 90 80 / 40 50 50 60